In recent years, the fringe field switching (FFS) mode has often been employed as the display mode of liquid crystal display devices for smartphones and tablet devices. A liquid crystal display device of the FFS mode is disclosed in Patent Document No. 1, for example.
A liquid crystal display device of the FFS mode includes a pair of electrodes for producing a fringe electric field on one of a pair of substrates sandwiching therebetween a liquid crystal layer of a horizontal alignment type. This pair of electrodes typically are a pixel electrode having a plurality of slits, and a common electrode placed under the pixel electrode with an insulating layer therebetween. When a voltage is applied between the pixel electrode and the common electrode, a fringe electric field is produced, and the alignment direction of the liquid crystal molecules change due to the alignment regulating force of the fringe electric field.
Thus, with a liquid crystal display device of the FFS mode, the alignment state of the liquid crystal molecules is controlled by using a fringe electric field. In the FFS mode, the liquid crystal molecules rotate in a plane that is parallel to the display surface, thereby realizing desirable viewing angle characteristics.
Patent Document No. 2 discloses a technique in which a flattening film having depressed portions and protruding portions is provided under the common electrode in order to improve the response speed in the FFS mode. Patent Document No. 2 states that such a structure gives the common electrode protrusions/depressions reflecting the surface shape of the flattening film, thereby allowing a stronger fringe electric field to be produced.